Ventilation in terrarium with computer fan?

[Fishkeeper]

I have a 12x12x20 Zilla terrarium, that I'm trying to rig for micro-mini orchids that like to be lightly and evenly moist. I'm having trouble keeping humidity nice and high without soaking the background too much, so I'm thinking about closing off some of the external ventilation. I'd like some thoughts.

Right now, it has a ventilation grate maybe 3/4" of an inch tall, running across the lower front of the tank. There's also a mesh strip about 1" wide, across the back of the lid, and a few slim gaps around the plastic covering the rest of the lid. A 3.5" computer fan is fastened to the underside of the ceiling, with some clearance above it, pointing down. It runs constantly, which is probably why the humidity keeps escaping. I don't want to shut it off, so I'm thinking, instead, I should cover some of the slit on the lid.

I don't suppose anyone else has a terrarium of roughly this size, with a constantly-running computer fan?

[Shadeflower]

Hi Fishkeeper,
I have turned an old aquarium into an orchid terrarium and I just prop the lid up a bit leaving a small gap with a small computer fan. Works fine. I keep an eye on humidity but I don't check it much.
I've had lots of different terrariums, some work better than others, it's great when humidity doesn't fluctuate too much and doesn't need checking or adjusting.
If humidity does run low then I just add a bowl of water in front of the fan but this is easier for me as my fan sits on the floor, would be a bit harder with the fan on the ceiling.

If it was me I would turn your fan around to the way it is now, the way it is now is that it is blowing onto the plants, for orchids passive airflow, ie pulling air away from the plants is just fine and what I prefer. It shouldn't make much difference but long term I have found a fan can dry plants more so the ones directly below the fan will end up drying more than on the outside.
You just want to gently circulate the air, not too much that leaves move but not too little that it gets too humid. With a fan running constantly you need far less ventilation holes than if there is no fan running as the fan is constantly agitating the air to move. So yes you can reduce the gaps. Just keep an eye on the plants, the roots, the pots, as humidity increases the pots and roots should stay hydrated longer between waterings.

[Fishkeeper]

The fan is currently pointing at the floor. The only plant in its path is a begonia, which I'm sure likes having some insurance against its leaves being wet. I might have to angle it towards the door, though, it seems to be managing to dry out part of the wall via reflected air current.

I think I need to redirect the spray nozzles a little. I'm trying to find an arrangement that mists the whole background semi-evenly, and keeps the rest of things also at least somewhat damp.

While I'm at it with the rejiggering and adjustments, anyone know how tolerant Lepanthopsis astrophora is of some water droplets on its leaves?

---------- Post added at 05:16 PM ---------- Previous post was at 03:49 PM ----------

I covered the ventilation strip on the top, so it only has about 1/3" of its width exposed. Nearly instant improvement. The glass is fogging a little, and the cork bark stays dampish, which looks much better. I put my hand in the tank and I feel the humidity right away. Will have to figure out where I put that hygrometer so I can get some numbers and start tuning things.

[Draikan]

IMO, constant internal circulation and slowly closing the passive vents until humidity is about right is a really great approach. The smallest tank I've run this way was 12*18*36, but it makes sense to me that a similar approach would be ok for smaller tanks too. I'm passionate about using internal circulation fans, I think they are great.

I've had best results from the fan either pointing up, or forward toward the glass. I'm not sure it matters too much, but as ShadeFlower said you probably don't want it pointed right at the plants. In my tanks, with my mister placement, up works best and helps the air move in the same direction as the mist during watering, which seems to help the mist get onto the plants and mounts.

I have multiple Lepanthopsis astrophora (this is one of my favorite plants), including some very small keikis, which I spray every other day until they are saturated. They don't mind it at all, no negative effects that I can see. With the internal circulation the leaves are dry by lights out. Without circulation I would be a bit more nervous about wet leaves. I'm only misting in the morning, never later in the day or at night, and I always give at least a day between waterings to dry off.

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[Fishkeeper]

That's really good to know about the L. astrophora. The spots where I think mine will look good and grow well all involve some water droplets getting on the plant. It dries before night, at least.

I'm trying for consistently high humidity, and for the background to be some reasonable degree of moist at all times. At the moment, it's cycling between "moist" and "slightly less moist". I have the sprayers running once in the morning for 15 seconds (which I think is a bit long), and then several other sprays of 3-5 seconds each. I might turn them all down somewhat, but I am trying to spray frequently.

That said, the goal is to hit the nicely moist circumstances that these plants like, with the minimum possible amount of actual spraying. Minimizes the water on the leaves of everything, which is particularly important given that I have a couple of begonias. One of them, the one right under the fan, is already pretty unhappy- I accidentally sprayed it directly while it was in a temporary holding box, without any active air movement, and a bunch of its leaves melted. Don't spray water directly on begonias that have no active air movement, they do not like it. That's part of why the fan is pointing downward, because I'm trying to keep it from getting melted any further. Once it's established and more robust against moisture, I might shift the fan a little, but right now I'm trying to protect this poor begonia.